It has long been commercially desirable to modify electrically conductive substrates, and particularly metal substrates, by introduction of normally solid metals and metalloids into a surface portion of the substrate, and this has heretofore been accomplished by, e.g., dipping procedures, by surrounding the substrate with a powder of the metal to be introduced and then heating in a furnace, and by other methods which are cumbersome, offer little real control of the introduction of the metal or metalloid, and are not practical when expensive metals such as tungsten are to be introduced. Introduction of metals into substrate surfaces has also been accomplished by use of the ion beam and the laser, but such approaches require expensive apparatus and very high power and in all events are limited to treatment of small substrate areas, as in the manufacture of microcircuits. Disadvantages of such prior-art methods have been overcome when the material to be introduced into the substrate is normally gaseous, as when nitrogen is to be introduced. Thus, ion nitriding has been accomplished when large substrate surface areas are to be treated, as described for example in "Nitriding, Sintering and Brazing by Glow Discharge", Claude K. Jones et al, Metal Progress, February, 1964. But there has been a continuing need for a method and apparatus which would make it possible to introduce metals and metalloids economically into a surface of the conductive substrate, especially when the substrate is of metal, or has a large surface area to be treated, or when the substrate surface to be treated is curved or irregular rather than flat, or when the metal to be introduced into the substrate has a high melting point or is expensive.